Mammalian target of Rapamycin (mTOR) is an atypical serine/threonine protein kinase, is a member of the phosphoinositide 3-kinase (PI3K) related kinase family, and is a main signaling molecule of cell functions such as intracellular synthesis and catabolism. The mTOR signaling pathway has a close relationship with nutrition, energy states and growth factors, and modulates many cellular processes including autophagy, protein, lipids and lysosomes synthesis, and energy metabolism, cytoskeleton organization, cell survival, and so on. Under the changing periphery nutritional conditions in mammalian cells, mTOR regulates the conversion between synthesis and degradation metabolism to enable the cells to grow and survive under different nutritional conditions. Because of the important role of mTOR in cells, aberrant or deregulated mTOR signaling can lead to human diseases (such as cancer and other diseases). Therefore, mTOR signaling pathway is becoming an important target for the design of anticancer drugs.
The activation of the PI3K/Akt/mTOR signaling pathway is closely related to a variety of tumorigenesis. mTOR can accelerate cell cycles, reduce apoptosis and promote tumor cell migration in brain glioma, breast cancer, and ovarian cancer. Activation of mTOR begins at several ligand-activated growth factor receptors on the cell surface, such as epidermal growth factor receptor and insulin-like growth factor 1 and -2 (IGF-1 and IGF-2). The activation of the receptors leads to the activation of the PI3K kinase, thereby resulting in the activation of the downstream effector Akt protein. Akt is a regulatory factor that can regulate cell survival in multiple levels. After phosphorylation, Akt inhibits the downstream TSC1/2 complex, and thus mTOR is activated by Rheb. In the downstream of the PI3K/Akt and PEN/Akt and Ras/Erk1/2 signaling pathways, the TSC1/2 complex plays a vital role in the regulation of mTOR activation.
It has been found two different mTOR protein complexes, mTORC1 and mTORC2, in a cell. Both of the protein complexes contain a unique protein interacting with mTOR, and are regulated by different mechanisms, respectively. Great progress has been made in the research and development of mTOR inhibitor drugs. Rapamycin is the first discovered mTOR inhibitor and has been shown a good tumor-inhibiting effect in a variety of cancer models. Although rapamycin analogues with better pharmacological properties have been developed, however, the rapamycin analogues available in clinically are only confined to a few cancers. The important discovery that Akt is an important kinase in the survival of cancer cells and mTORC2 can directly phosphorylate Akt provides a new way of thinking in the research of anti-cancer with mTORC2, but also contributes to the research and development of the second-generation anti-cancer drugs which act on both mTORC1 and mTORC2 targets simultaneously. Simultaneous inhibition of the activities of both mTOR complexes (mTORC1 and mTORC2) in cancer cells provides a wider and more effective anti-cancer effect.
mTORC1 has six subunits, and mTORC2 is composed of seven subunits. Among them, mTOR, mLST8, DEPTOR and Tti1/Tel2 catalytic subunits are present in mTORC1 complex and mTORC2 complex. The two complexes have different regulatory proteins, Raptor and PRAS40 present in the mTORC1, rictor, mSin and protor1/2 present in the mTORC2. Upstream signals of mTORC1 are mainly from the intracellular and extracellular pathways, including growth factors, cell stress, energy status, oxygen and amino acids. These signals control many of the major processes in the cell including autophagy and synthesis of proteins, mRNA, and lipid. Heterodimer (TSC1/TSC2) is a key upstream regulatory factor of mTORC1, with a function of being an activating protein of the Rheb GTPase. Rheb coupled to GTP directly interacts with mTORC1 and activates its enzymatic activity. As a GTPase activating protein of Rheb, TSC1/2 converts Rheb to a status without activity and coupled to GDP by a negative regulation. mTORC1, after phosphorylation, activates its downstream factors 4E-BP1 and S6K1 to promote protein expression and to increase the generation of mRNA. Furthermore, mTORC1 controls cellular metabolism and ATP generation in combination with the SREBP1/2 transcription factor and HIF1-alpha. In addition to the role in anabolism, mTORC1 can also modulate autophagy by negative regulations in order to promote cell growth. In mammals, mTORC1 directly phosphorylates the ULK1/Atg13/FIP20 kinase complex and inhibits the initiation of autophagy. mTORC1 can also affect the autophagy by other mechanisms, such as modulating the inhibiting factor of autophagy, DAP1, and promoting the formation of the lysates.
Compared with mTORC1, people have a less knowledge of the mTORC2 signaling pathway. mTORC2 signaling is not sensitive to nutrient conditions but has a response to some growth factors. mTORC2 regulates several members in the AGC kinase subfamily, such as Akt, SGK1 and PKC-α. Akt activates the downstream signaling proteins to regulate cell metabolism, survival, apoptosis, growth and proliferation. mTORC2 directly phosphorylates the Akt (Ser473) site to activate its function. But in the absence of mTORC2, the phosphorylations of TSC2 and GSK3-β are not affected. mTORC2 can also directly activate the SGK1 kinase to regulate ion transfer and cell growth. However, compared with Akt, the function of SGK1 is completely inhibited in the absence of mTORC2. mTORC2 activated PKC-α can affect the formation of the actin cytoskeleton.
Many studies have shown that mTOR signaling pathway is related to the development of cancers. Many components between the downstream of PI3K and the upstream of mTOR mutate in cancers, including Tsc1/2, Lkb1, Pten and Nf1. The activation of oncogenes of mTOR can induce the growth, survival and proliferation processes of several cancer cells. More and more researches show that uncontrolled protein expression is related to mTORC1. Because 4E-BP1/eIF4m at downstream of TORC1 plays a key role in tumor formation. 4E-BP1/eIF4 transfers the oncogenic signals from the mRNA expression by Akt. These signals lead to the expression of several special oncogenic proteins, and finally the oncogenic proteins regulate the cell survival, cell cycle, neovascularization, energy metabolism and tumor metastasis. In addition, ribosome biogenesis associated with the mTOR activation may be related to the high levels of cell growth.
The increase of the lipid synthesis is an important sign of tumor cell proliferation. This is because that the new cells need to generate fatty acids to synthesize the cell membranes. PI3K signaling pathway activates the lipotropic synthesis factor (SREBP1), while mTORC1 is a signaling factor for the activation of SREBP1 by PI3K. Meanwhile SREBP1 also drives the expression of some pentose phosphate oxidative constituent factors. The pentose phosphate oxidative pathway controls the lipid synthesis and nucleic acid synthesis.
Autophagy can be very strongly suppressed by the continuous activation of PI3K/mTORC1 signaling. To tumor cells, the disadvantage of suppressing autophagy is the reduced survival ability of tumor cells under conditions of lack of nutrition and energy, thereby affecting the formation of tumor.
mTORC2 has been confirmed to have the potential to control the formation of the vascular system and immune chemokines. This indicates that the inhibition of mTORC2 can impair the formation and sustainable growth of tumors by preventing angiogenesis or reducing invasion of immune cells. In some tumors, the high expression of mTORC2 is associated with the high expression of its subunits rictor. The deletion of PTEN, a tumor suppressor gene, leads to an enhanced function of TORC2 in mice. These results support that mTORC2 plays an important role in tumorigenesis, and meanwhile show that reducing of mTORC2 vitality may potentially have an important significance in the anticancer therapy.